1. Field of the Invention
The present invention relates to a brushless motor. More particularly, the present invention relates to a brushless motor used as a driving source of an industry robot, a machine tool, an electric car or an electric train.
2. Description of the Related Art
In order to miniaturize a motor and to increase output power and torque thereof, it is important that an energy density Edc is high, which implies a ratio of the volume of the motor to the output power. Moreover, in order to simplify the structure of the motor, it is important to minimize the number of slots for winding arrangement and make a working efficiency of a winding operation higher.
Such a brushless motor is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 11-98791). As shown in FIG. 1, the known brushless motor is a surface magnet type brushless DC motor including 14 poles and 12 slots. The brushless motor is provided with: a group of permanent magnets 102 placed on a cylindrical surface of a rotor 101 in which 14 poles are arranged in series; and a stator 104 in which 12 slots 103-1 to 103-12 are radially placed on the same circumference at a same angular interval. One set of windings 105-U1, 105-V1 and 105-W1 and another set of windings 105-U2, 105-V2 and 105-W2, which respectively positionally correspond to each other, are placed at positions in which phases are mutually shifted counter-clockwise by an electric angle of 120 degrees, in six pairs of slots, each of which is composed of two slots adjacent to each other, among 12 slots 103-1 to 103-12. Moreover, six windings 105-U1′, 105-V1′, 105-W1′, 105-U2′, 105-V2′ and 105-W2′ are respectively placed such that they are shifted by a rotational angle of 30 degrees with respect to the six windings 105-U1, 105-V1, 105-W1, 105-U2, 105-V2 and 105-W2. A U-phase voltage having a phase of 0 is provided for the winding 105-U1 and the windings 105-U2, 105-U1′ and 105-U2′. A V-phase voltage having a phase delayed by about 120 degrees from that of the U-phase voltage is provided for the windings 105-V1, 105-V2, 105-V1′ and 105-V2′. A W-phase voltage having a phase delayed by about 120 degrees from that of the V-phase voltage is provided for the windings 105-W1, 105-W2, 105-W1′ and 105-W2′.
An output torque T of the known brushless motor is given by the following equation:T=p{φ·Ia·cos (β)+(Lq−Ld)Ia2·sin(2β)/2}.  (1)Here,                p: Number of Pole Pairs (Number of Poles/2)        φ: Maximum armature flux linkage of the permanent magnet        Ia: Armature current        β: Phase of armature current        Ld: Direct-axis inductance (Inductance in the d-axis direction)        Lq: Quadrature-axis inductance (Inductance in the q-axis Direction)The phase of the armature current is defined under the assumption that the phase of the U-phase voltage is 0. The first term on the right side of the equation (1) represents a magnet torque, and the second term on the right side represents a reluctance torque.        
In the above-mentioned surface magnet type brushless motor, in which the permanent magnet is placed on the surface of an iron core 101, the following equation:Lq≈Ld,  (2)can be established from the property of that structure. Here, the symbol “≈” indicates that the Lq is approximately (substantially or nearly) equal to the Ld.
Thus, the output torque of the surface magnet type brushless motor is substantially given by the following equation:T=p{φ·Ia·cos(β)}.Accordingly, the output component represented by the second term on the right side of the previous equation is 0. That component is not outputted. The surface magnet type brushless motor can effectively use only the magnet torque indicated by the first term on the right side of the equation (1). Hence, the increase in the energy density is suppressed.
It is desirable to increase the energy density by effectively using the reluctance torque indicated by the second term on the right side of the equation (1).